In the titration of 250 cm3 of 2M HCL, which of the following bases
requires the least volume:
a.  2.0M NAOH
b. 0.50M Al(OH)3
c. 1.5M Ca(OH)2
d. 1.0M Mg(OH)2

One can simply use the formula V(a)C(a) = V(b)C(b), where V(a) is the
volume of the acid, C(a) is the concentration of the acid, V(b) is the
volume of the base, C(b) is the concentration of the base.

In this particular example we need to note that the base is actually
talking about the concentration of the [OH-] ion concentration.

a. will produce 2.0M OH-
b. will produce 1.5M OH-
c. will produce 3.0M OH-
d. will produce 2.0M OH-

One can plug in the values and find the corresponding lower volume,
or...

By inspection, one can see that the higher the concentration of OH-
the lower the volume that is needed to titrate the solution. 
Consequently, choice C, will require the least volume.

That volume is (2/3)*250ml.